Sulfur is an objectionable element which is nearly ubiquitous in fossil fuels. The presence of sulfur has been correlated with corrosion of pipeline, pumping, and refining equipment, and with premature breakdown of combustion engines. Sulfur also contaminates or poisons many catalysts which are used in the refining and combustion of fossil fuels. Moreover, the atmospheric emission of sulfur combustion products such as sulfur dioxide leads to the form of acid deposition known as acid rain. Acid rain has lasting deleterious effects on aquatic and forest ecosystems, as well as on agricultural areas located downwind of combustion facilities. Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389. Regulations such as the Clean Air Act of 1964 require the removal of sulfur, either pre- or post-combustion, from virtually all fossil fuels. Conformity with such legislation has become increasingly problematic due to both the rising need to utilize lower-grade, higher-sulfur fossil fuels as clean-burning, low-sulfur petroleum reserves become depleted, and as the progressive reductions in sulfur emissions required by regulatory authorities become more stringent. Monticello, D. J. and J. J. Kilbane, "Practical Considerations in Biodesulfurization of Petroleum", IGT' s 3d Intl. Symp. on Gas, Oil, Coal, and Env. Biotech., (Dec. 3-5, 1990) New Orleans, La.
There are several well-known physicochemical methods for depleting the sulfur content of fossil fuels prior to combustion. One widely-used technique is hydro-desulfurization, or HDS. In HDS, the fossil fuel is contacted with hydrogen gas at elevated temperature and pressure, in the presence of a catalyst. The removal of organic sulfur is accomplished by reductive conversion of sulfur compounds to H.sub.2 S, a corrosive gaseous product which is removed by stripping. As with other desulfurization techniques, HDS is not equally effective in removing all forms of sulfur found in fossil fuels. Gary, J. H. and G. E. Handwerk, (1975) Petroleum Refining: Technology and Economics, Marcel Dekker, Inc., New York, pp. 114-120.
For example, HDS is not particularly effective for the desulfurization of coal, wherein inorganic sulfur, especially pyritic sulfur, can constitute 50% or more of the total sulfur content of the fossil fuel the remainder being various forms of organic sulfur. Pyritic sulfur is not efficaciously removed from fossil fuel by HDS. Thus, only a fraction of the total sulfur content of coal may be susceptible to removal by physiochemical methods such as HDS. The total sulfur content of coal can typically be close to about 10 wt % or it can be as low as about 0.2 wt %, depending on the geographic location of the coal source.
HDS is relatively more suitable for desulfurizing liquid petroleum, such as crude oil or fractions thereof, as close to 100% of the sulfur content these fossil fuels can be organic sulfur. Crude oils can typically range from close to about 5 wt % down to about 0.1 wt % organic sulfur; crude oils obtained from the Persian Gulf area and from Venezuela can be particularly high in sulfur content. Monticellow, D. J. and J. J. Kilbane, "Practical Considerations in Biodesulfurization of Petroleum", IGT's 3d Intl. Symp, on Gas, Oil, Coal, and Env. Biotech., (Dec. 3-5, 1990) New Orleans, La., and Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389.
Organic sulfur in both coal and liquid petroleum fossil fuels is present in a myriad of compounds, some of which are labile and can be readily divested of sulfur by HDS, and some of which are refractory and do not yield to HDS treatment. Shih, S. S. et al., (1990) AIChE Abstract No. 264B (complete text available upon request from the American Institute of Chemical Engineers); hereinafter, Shih et al. Thus, even HDS-treated fossil fuels must be post-combustively desulfurized using an apparatus such as a flue scrubber. Flue scrubbers are expensive to install and difficult to maintain, especially for small combustion facilities. Moreover, of the sulfur-generated problems noted above, the use of flue scrubbers in conjunction with HDS is directed to addressing environmental acid deposition, rather than other sulfur-associated problems, such as corrosion of machinery and poisoning of catalysts.
The classes of organic molecules which are often labile to HDS treatment include mercaptans, thioethers, and disulfides. Aromatic sulfur-bearing heterocycles (i.e., aromatic molecules bearing one or more non-carbon atoms on the aromatic ring itself) comprise the major class of organic sulfur molecules retractory to HDS or similar physicochemical treatments. These refractory molecules typically require desulfurization conditions harsh enough to degrade valuable hydrocarbons in the fossil fuel. Shih et al.
These significant drawbacks to HDS are typical of physicochemical desulfurization methods generally. As a result, there has been considerable interest in the industry for at least the past 20-30 years in developing commercially viable techniques of microbial desulfurization, or MDS. MDS is generally described as the harnessing of metabolic processes of suitable bacteria to the desulfurization of fossil fuels. MDS typically involves mild (e.g., physiological) conditions, and does not involve the extremes of temperature and pressure required for HDS. Several species of chemolithotrophic bacteria have been investigated in connection with MDS development, due to their abilities to metabolize the forms of sulfur generally found in fossil fuels. For example, species such as Thiobacillus ferrooxidans are capable of extracting energy from the conversion of pyritic (inorganic) sulfur to water-soluble sulfate. Such bacteria are envisioned as being well-suited to the desulfurization of coal. Other species, including Pseudomonas putida, are capable of catabolizing the breakdown of organic sulfur molecules, including to some extent sulfur-bearing heterocycles, into water-soluble sulfur products. However, this catabolic desulfurization is merely incident to the utilization of the hydrocarbon portion of these molecules as a carbon source: valuable combustible hydrocarbons are lost. Moreover, MDS proceeds most readily on the same classes of organic sulfur compounds as are most susceptible to HDS treatment. Thus, although MDS does not involve exposing the fossil fuels to the extreme conditions encountered in HDS, a significant amount of the fuel value of the coal or liquid petroleum can be lost, and the treated fuel often still requires post-combustion desulfurization. Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389, and Hartdegan, F. J. et al., (May 1984) Chem. Eng. Progress 63-67.
A need remains to develop more effective methods for pre-combustion desulfurization. This need grows progressively more urgent as lower-grade, higher-sulfur fossil fuels are increasingly used, while concurrently the sulfur emissions standards set by regulatory authorities become ever more stringent.